PROJECT SUMMARY The assembly of individual neurons into interconnected networks is essential for sensation of external stimuli and induction of a behavioral response. When this circuitry is disrupted as in chronic pain and peripheral neuropathies, the effects can have a debilitating effect on quality of life. The majority of pain research has focused on functional properties of the peripheral somatosensory neurons, but the factors that drive the appropriate connectivity of these neurons to central interneurons could also lead to new avenues for treatment. In the brain, there is a dense neuropil of incoming neuronal axons and receptive neuronal dendrites, yet not all neurons can communicate with one another. For instance, sensory neurons responsive to light touch must identify distinct partner neurons from sensory neurons responsive to harsh touch. Thus, there is high impetus to promote selective neuronal partner matching, which ensures that a given sensory input will lead to a predictable, appropriate output response. Despite this centrality to ensuring reproducible reactions, little is known about how neurons select specific partners for synapse formation. A better understanding of developmental and molecular mechanisms that drive synaptic partner choice could help to uncover processes that are disrupted and driving the etiology of some pain neuropathies. This proposal aims to provide an inroad to initiate discovery of developmental mechanisms and genes that are involved in partner choice, and that could be key to revealing the molecular logic used in neuronal connectivity. This proposal will investigate connectivity in a compact nervous system, that of the fruit fly Drosophila melanogaster, due to multiple advantages including genetic control of individual neurons, stereotyped connectivity, and methods to assess synaptic partner choice. Although separated evolutionarily by hundreds of millions of years, about 75% of genes are functionally conserved from D. melanogaster to humans, and it is likely that even if the genes identified in D. melanogaster partner choice do not have human homologs, the overall logic of how neurons select partners will be preserved across these species. Recent discoveries have begun to define a network of neurons of known connectivity in the D. melanogaster nervous system, and this study will focus primarily on two partner neurons, using additional neurons in the network to test questions of generalizability to other synaptic partners. In the first aim, functional mechanisms for establishing specificity among numerous potential partners will be interrogated. In the second aim, the individual molecules that mediate this synaptic recognition will be explored. Together, these two complementary developmental and molecular sets of experiments will allow the testing of the overall hypothesis that a combinatorial code of adhesion molecules instructs specificity in synaptic partner matching. !